1. Field of the Invention
The present invention relates to a control circuit of a direct current-direct current (DC--DC) convertor to be used as a power source for various electronic apparatuses. More particularly, it is related to a control circuit of a DC--DC convertor to be used for keeping constant the value of a voltage which is supplied to a hand-held-type electronic apparatus such as a notebook-type personal computer or the like, even if the voltage of a battery installed in the power source of this apparatus varies.
2. Description of the Related Art
In a hand-held type electronic apparatus such as a notebook-type personal computer or the like, a stepdown-type DC--DC convertor is used for keeping the voltage to be supplied to this apparatus constant, even if the output voltage of the battery installed in this apparatus reduces as it progressively discharges.
FIG. 1 is a schematic diagram showing a step-down DC--DC convertor and its control circuit. In this figure, power supplied from an input power source 1 to a load 5 is controlled by the DC--DC convertor. A capacitor 2 (C2) smooths a rectangular waveform current which is generated by the ON/OFF operation of transistor Q1, but this capacitor is not required if the output impedance of the input power source 1 is sufficiently small. A transistor 3 (Q1) is a main transistor to be used as the switch in the DC--DC convertor. Power to be supplied to the load 5 is adjusted by controlling an ON time of the transistor 3 using, for example, a PWM (Pulse Width Modulation) control. An inductor 4 (L1) is a smoothing reactor, and a capacitor 6 (C1) is a smoothing capacitor. A diode 7 (D1) is a flywheel diode used for rectification.
During the ON time of the transistor 3, power outputted from the input power source 1 is supplied to the load 5 and the capacitor 6 through the smoothing reactor 4. During the OFF time of the transistor 3, power is not supplied from the input power source 1 to the load 5. Instead, power is supplied to the load 5 by supplying the energy stored in the smoothing reactor 4 by the capacitor 6 and the flywheel diode 7.
In FIG. 1, the control circuit of the DC--DC convertor is basically composed of an amplifier 10 for amplifying an error voltage, a comparator 11, a sawtooth wave generator 12, and a driver 13.
The amplifier 10 amplifies the difference between a predetermined reference voltage Vref and a feedback voltage from the output of the DC--DC convertor.
The comparator 11 has three inputs. The output from the amplifier 10 and a soft start (SS) voltage (the terminal voltage of a capacitor 15) are respectively inputted to two non-inverting inputs among the three inputs. The output of the sawtooth wave generator 12 is inputted to the inverting input. The comparator 11 compares the input having the lowest voltage value between the two non-inverting inputs with the inverting input. Then the comparator 11 outputs H when the value of the non-inverting input having the lowest voltage is larger than the value of the inverting input, and outputs L in the reverse case.
A terminal voltage of the capacitor 15 which is charged by a current outputted from a constant current source 14 is supplied as an SS voltage. When the DC--DC convertor is activated, both ends of the capacitor 15 are briefly connected by a switching circuit, not-shown in FIG. 1. Accordingly, the SS voltage increases from 0 with a constant gradient from when the convertor is activated.
The output of the comparator 11 is supplied to a gate terminal of the main transistor 3 of the DC--DC convertor through the driver 13. Since the transistor 3 is an n-channel-type FET, it becomes ON while the output of the driver 13 is H, that is, the comparator 11 is outputting H. During this period, current is supplied from the input power source 1 to a load 5 side.
FIG. 2 is a time-chart explaining the operations of the DC--DC convertor control circuit shown in FIG. 1. In this figure, operations of the control circuit at the activation time of the DC--DC convertor are explained. In FIG. 1, an SS voltage for a soft start is inputted to one of the two non-inverting inputs of the comparator 11. First, a case where an input terminal for the SS voltage is not provided is assumed. In FIG. 2, since the output of the DC--DC convertor, that is, the voltage to be supplied to the load 5 is 0 at the moment of the activation of the convertor when t=0, an input voltage to be inputted to the inverting input terminal of the amplifier (error amplifier) 10 for amplifying an error voltage is 0, and the output of the amplifier 10 becomes the maximum. As time elapses, the output voltage of the convertor rises, and accordingly, the output of the amplifier 10 gradually lowers. When the input terminal for the SS voltage is not provided, the comparator 11 compares the output of the amplifier 10 with that of the sawtooth wave generator 12. Then, the comparator 11 outputs H while the output of the amplifier 10 is greater than that of the sawtooth wave generator 12, and this output becomes the signal for turning on the transistor 3 through the driver 13. Therefore, when the input terminal for the SS voltage is not provided, the ON time of the transistor 3 becomes long immediately after the activation of the convertor, so the ON duty becomes 100%. As a result, a high input current flows into the transistor 3 and the smoothing reactor 4. In order to prevent this flow of high input current, the SS voltage is inputted to the comparator 11.
The comparator 11 compares the value of the signal having the lowest voltage between the signals inputted to the two non-inverting input terminals, with the value of the sawtooth wave signal supplied to the inverting input terminal. Then, the comparator 11 outputs H only while the non-inverting input signal having the lowest voltage value is greater than the sawtooth wave signal. The SS voltage, that is, the terminal voltage of the capacitor 15, gradually rises with time as shown in FIG. 2, and the output of the comparator 11, that is, the gate control signal of the transistor 3, becomes H only while this terminal voltage is higher than the sawtooth wave voltage. At time tc, the output voltage of the amplifier 10 is equivalent to the terminal voltage of the capacitor 15. At or after this time, the output of the comparator 11 becomes H while the output of the amplifier 10 is higher than the sawtooth wave voltage. Thus, at the activation time of the convertor, a soft start operation in which the output of the comparator 11 becomes H, that is, the ON time of the transistor 3 gradually becomes longer, is realized.
In the circuit shown in FIG. 1, as explained in detail by the inventor in a prior filed application, the point where the output of an amplifier for amplifying a voltage error and the rising terminal voltage of the soft start capacitor cross, that is, the time tc shown in FIG. 2, depends on the amount of the load on the DC--DC convertor. Further, as the load becomes smaller, the time tc becomes shorter, and as the load becomes bigger, the time tc becomes longer. Therefore, the rising output characteristic of the DC--DC convertor could not be controlled by the capacitance of the soft start capacitor 15.
Prior filed application: Tokkaihei No. 9-154275 (Tokuganhei No. 7-308856) "Direct current-direct current conversion control circuit and direct current-direct current conversion apparatus"
FIG. 3 is a circuit diagram of the second conventional example of a DC--DC convertor and its control circuit. Since the operations of this circuit were explained in the above prior filed application, only an outline of this application will be explained below.
When FIG. 3 is compared with FIG. 1, there are some basic differences. That is, while a voltage error amplifier 20 has three inputs, a comparator 21 has two inputs. Further, synchronous type of rectification is adopted. According to this type, a synchronous rectification transistor 8 (Q2, an n-channel-type FET) is connected in parallel with the flywheel diode 7 on a DC--DC convertor side, and a driver 24 for supplying an OFF/ON signal to the gate terminal of the transistor 8 is added.
In FIG. 3, two non-inverting inputs are provided to the voltage error amplifier 20. To one of the two inputs, the soft start voltage supplied to the comparator 11 as shown in FIG. 1 is inputted. By contrast, the comparator 21 compares the output of the amplifier 20 and that of the sawtooth wave generator 22, and supplies a non-inverting output to a driver 23 while the output of the amplifier 20 is greater than the sawtooth wave voltage outputted from the sawtooth wave generator 22. The output signal of the driver 23 is supplied to the gate terminal of the transistor 3. An inverting output of the comparator 21 is supplied to the gate terminal of the transistor 8 through the driver 24. When the output signal of the driver 24 is H, the transistor 8 turns on. The transistor 8 turns on while the transistor 3 turns off. At this time, since power is supplied to the load 5 by flowing the energy stored in the smoothing reactor 4 through the capacitor 6 and the transistor 8, the loss caused by the flywheel diode 7 can be reduced. Therefore, this transistor 8 is called a synchronous rectification transistor.
Thus, by inputting the soft start voltage to the voltage error amplifier 20 as shown in FIG. 3, not by inputting it to the comparator 11 as shown in FIG. 1, the dependence of the rising characteristic of an output voltage at the activation time of the DC--DC convertor on the load on the convertor is removed, so that this rising characteristic can be controlled only by changing the constant of the soft start capacitor 15. The detail of this process is omitted since it was fully explained in the above-mentioned prior filed application, and further it is not directly related to the contents of the present invention.
In FIG. 3, however, there arises a new problem that at the activation time of the convertor, an oscillating current (resonance current) flows into the smoothing reactor 4, the smoothing capacitor 6, and the synchronous rectification transistor 8, so that the output voltage of the convertor instantaneously becomes excessive, and the load may be destroyed in such a case.
FIG. 4 is a timechart explaining this problem. In this chart, after a start-up instruction for the DC--DC convertor is supplied, the capacitor 15 for a soft start begins to be charged from the constant current source 14, and the terminal voltage (SS voltage) of the capacitor 15 linearly rises. At this time, an input voltage to the inverting input terminal of the voltage error amplifier 20, that is, the output voltage of the DC--DC convertor, rises corresponding to the gain Rc/Rin of the amplifier. When the output voltage rises up to the point to cross the sawtooth wave generated by the sawtooth wave generator 22, the output of the comparator 21 becomes H, and an ON signal is supplied to the main transistor 3 through the driver 23.
In respect of the pulse widths causing the main transistor (Q1) 3 to turn on, the second pulse has a wider width than that of the first pulse, as shown in FIG. 4. In FIG. 4, however, only two pulses generated immediately after the activation are shown. Generally, however, a plurality of pulses are generated immediately after the activation, and the pulse width is gradually wider with time. As a result, the output voltage of the DC--DC convertor gradually rises. At the point where this output voltage becomes equivalent to the terminal voltage (SS voltage) of the soft start capacitor, the amplifier 20 intends to immediately reduce its own voltage output. However, in the case where the gain of the amplifier 20 is high, it is necessary to insert a capacitor Cc in series with resistor Rc to delay the phase of the amplifier 20, thereby preventing the oscillation of the amplifier 20. By doing so, the output voltage of the amplifier 20 overshoots, and the amplifier 20 continues to be ON excessively for the period of time t2 as shown in FIG. 4.
As a result, the output voltage of the DC--DC convertor further rises, and becomes greater than the terminal voltage of the soft start capacitor 15. Then, the output voltage of the amplifier 20 reduces, and becomes smaller than the sawtooth wave outputted from the sawtooth wave generator 22. Accordingly, the non-inverting output of the comparator 21 becomes L, and the inverting output thereof becomes H, thereby turning off the main transistor 3, and turning on the synchronous rectification transistor 8 through the drivers 23 and 24, respectively.
Here, when the synchronous rectification transistor 8 turns on, the energy stored in the smoothing reactor 4 flows along a route which passes through the smoothing capacitor 6 and the synchronous rectification transistor 8 during the excessive ON time of the main transistor 3, that is, time t2. At this time, a current oscillation (resonance) caused by a smoothing reactor L and a smoothing capacitor C is generated, and during an OFF time of the main transistor 3, the output voltage of the convertor immediately rises shown as an inverting input terminal voltage of the amplifier 20 in FIG. 4. Therefore, in a worst case, there might arise the problem that the output voltage of the convertor exceeds the maximum rating of the load, thereby destroying the load.
In order to remove such a phenomenon, the gain of the amplifier 20 is reduced, and the overshooting as shown in FIG. 4 is not generated in the output voltage of the amplifier, so that the oscillation of the output voltage of the DC--DC convertor can be prevented. In this case, however, there is a problem that since the response characteristic of the amplifier deteriorates, the response characteristic of the convertor also deteriorates when the load changes rapidly.